The invention relates generally to data storage. More specifically, the invention relates to a method and apparatus for simultaneously reading and writing data to quasi-random access memory.
A typical computer may use different types of memory for storing data. It may use random access memory such as DRAMs and SRAMs. It may use sequential memory such as magnetic tape. It may use quasi-random access memory such as "read/write" optical discs.
Read/write optical discs include discs that allow new data to be written only once and discs that allow new data to be written many times. A DVD+RW disc is one type of read/write disc that allows new data to be written many times.
Read/write drives can write data to read/write optical discs. A typical read/write drive includes an optical pickup unit that is moved radially along the disc. The optical pickup unit, in turn, includes a laser, an optics assembly (including a focusing lens) and a photodetection system.
A read/write drive typically has a read mode of operation and at least one of the following write modes of operation: a write-append mode and an insert-edit mode. The write-append mode allows new data to be appended to previously written data on the read/write disc, and the insert-edit mode allows previously written data to be overwritten with new data.
During read and write operations, the optical pickup unit is moved to a target location where data is to be read or written. Movement to the target location may be characterized as a "short seek" or a "long seek." Short seeks are typically performed by displacing the focusing lens via an electro-mechanical actuator. Long seeks are typically performed by displacing the entire optical pickup unit via a sled assembly. Seeks performed via the electro-mechanical actuator are performed much faster than seeks performed via the sled assembly.
Data can be written "simultaneously" to the optical disc by interleaving the read and write operations. For instance, read and write operations may be performed simultaneously by reading data at a first location, performing a seek to a second location, writing data at the second location, performing a seek to a third location, reading data at the third location, performing a seek to a fourth location, writing data at the fourth location, and so on. Whether the electro-mechanical actuator or sled assembly is used for the seek depends upon the distance to the target location.
A latency time is associated with each seek operation. Since performing a seek via the actuator is much faster than performing a seek via the sled assembly, long seeks typically have much higher latency times than short seeks. The much longer latency times of the long seeks can make it difficult to perform interleaved read and write operations.
There is a need to minimize the latency times while performing interleaved read and write operations.